hw/arm/aspeed: Split G220A machine into a separate source file for better maintenance

hw/arm/aspeed: Split G220A machine into a separate source file for better maintenance

This commit refactors the Bytedance G220A BMC machine by moving its
implementation from aspeed.c into a new dedicated file
aspeed_ast2500_g220a.c.

The goal is to improve modularity and maintainability of Aspeed
machine definitions by isolating each platform into its own source
file. This aligns with ongoing efforts to simplify aspeed.c,
which previously contained all machine configurations.

Key updates include:

- Moved G220A_BMC_HW_STRAP1 definition and
g220a_bmc_i2c_init() function into aspeed_ast2500_g220a.c.
- Moved aspeed_machine_g220a_class_init() and related type
registration to the new file.
- Added the new file to the build system (meson.build).
- Removed all G220A-specific code from aspeed.c.

No functional changes.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20251104031325.146374-7-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>